Hamiltonian paths and cycles in hypertournaments

Given two integers n and k, n ≥ k > 1, a k-hypertournament T on n vertices is a pair (V, A), where V is a set of vertices, |V | = n and A is a set of k-tuples of vertices, called arcs, so that for any k-subset S of V , A contains exactly one of the k! k-tuples whose entries belong to S. A 2-hypertournament is merely an (ordinary) tournament. A path is a sequence v1a1v2a2v3...vt−1at−1vt of distinct vertices v1, v2, ..., vt and distinct arcs a1, ..., at−1 such that vi precedes vi+1 in ai, 1 ≤ i ≤ t − 1. A cycle can be defined analogously. A path or cycle containing all vertices of T (as vi’s) is Hamiltonian. T is strong if T has a path from x to y for every choice of distinct x, y ∈ V . We prove that every k-hypertournament on n (> k) vertices has a Hamiltonian path (an extension of Redei’s theorem on tournaments) and every strong k-hypertournament with n (> k + 1) vertices has a Hamiltonian cycle (an extension of Camion’s theorem on tournaments). Despite the last result, it is shown that the Hamiltonian cycle problem remains polynomial time solvable only for k ≤ 3 and becomes NP-complete for every fixed integer k ≥ 4.